The present invention relates to solid state radiation detectors and processes of manufacturing the same. More particularly, the present invention relates to an improved platinum-cadmium sulfide Schottky barrier photovoltaic detector and process of fabrication.
There exists a demand in high technology optical systems, such as in the optical guidance systems for radiation seeking missiles, for a high quantum efficiency, short-time response, solid state radiation detector. In these systems, such a detector must have a high response to near ultraviolet (UV) radiation, but must be substantially insensitive to radiation in the visible spectrum.
Silicon photodiodes have been proposed for these applications. However, such photodiodes are sensitive to the visible spectrum through approximately 8000 angstroms, and therefore they must be used in association with optical filtering to remove the visible radiation. Additionally, these detector-filter combinations have a relatively low quantum efficiency, e.g. 30% or less.
Cadmium sulfide based solid state radiation detectors have been utilized; however, the known characteristics of cadmium sulfide detectors suggest that they are useful primarily in visible range, and that they are relatively insensitive to ultraviolet radiation. It is desirable to provide a solid state radiation detector sensitive to near ultraviolet and short wavelength visible radiation and transparent to infrared radiation. This is particularly true if such a detector can be fabricated to have a high quantum efficiency and a relatively small optically active area so that it may be utilized in conjunction with the high resolution optics. Good infrared (IR) transmission characteristics enable the detector to be utilized in association with an IR sensor to produce a coaxial transducer suitable for use in association with unfiltered, high-resolution optics.
Recently cadmium sulfide based Schottky barrier diodes have been proposed as radiation detectors. Broadly speaking, a Schottky barrier diode is a junction diode in which the junction is formed between a semiconductor material and a metal contact, rather than between dissimilar semiconductor materials or carrier types, as in the case of an ordinary PN diode. The present invention constitutes an improvement over that disclosed in U.S. Pat. No. 4,000,502 owned by the assignee of the present application. Where applicable, the disclosure of that patent is specifically incorporated herein by reference. Disclosed therein is a platinum-cadmium sulfide Schottky barrier photovoltaic detector fabricated with the ohmic and barrier contacts located on the opposite sides of the cadmium sulfide substrate. This patented detector exhibits high quantum efficiency in the UV spectrum with good IR transmission characteristics.
The improved detector of the present invention is more amenable to production fabrication and quantity assembly techniques than the detector of U.S. Pat. No. 4,000,502. Rather than having ohmic and barrier contact connections on opposite sides of the device, the new detector is fabricated with both contacts on the front face. Organic insulating layers made of photoresist have been replaced by a hard, inorganic insulating layer. Furthermore, the copper and indium metalizations have been replaced with gold, titanium, nichrome and other metallizations. These improvements facilitate wire attachment by high-speed bonding techniques which are more reliable and which reduce the time and effort necessary for forming electrical contacts on the device. Furthermore, the overall reliability of the device is enhanced. The manufacturing process results in a compensated layer at the surface of the cadmium sulfide substrate which protects the device when excessive voltage is applied to it.